I bet they forgot to use the condoms and that will be a big plot point since Kaguya is having power struggles withing her family, and he was planning to go away for years. It may change everything, even if there is no actual pregnancy, but the scare and concept of it will be enough.
oh hmm latest Kaguya-Sama Miyuki is such a good guy. Kaguya wanting to progress further only cause she feels rushed due to circumstances, but not actually being ready and Miyuki being the upstanding guy not taking advantage of that and doing what she really wants (and then they do it anyways lol)||